In recent years thermotropic liquid crystalline polymers have become known to those skilled in polymer technology. Thermotropic liquid crystalline polymers are recognized to be capable of forming an anisotropic melt phase. Briefly, polymers of this type are capable of assuming a parallel ordering of the polymer molecular chains in the melt even under static conditions. These polymers commonly are prepared from monomers which are generally long, flat and fairly rigid along the long axis of the molecule and commonly have chain-extending linkages that are either coaxial or parallel.
The anisotropic character of such polymers in the melt may conveniently be confirmed by polarized light techniques whereby crossed polarizers are utilized. More specifically, the anisotropic melt phase may be observed by the use of a Leitz polarizing microscope at a magnification of 40.times. with the molten sample on a Leitz hot stage and under a nitrogen atmosphere. The anisotropic polymer melt transmits light under such conditions.
Thermotropic liquid crystalline polymers (as defined) include but are not limited to wholly aromatic polyesters, aromatic-aliphatic polyesters, aromatic polyazomethines, aromatic polyester-carbonates and wholly or non-wholly aromatic polyesteramides. The wholly aromatic polymers are considered to be "wholly" aromatic in the sense that each moiety present in the polymer contributes at least one aromatic ring to the polymer backbone and which enables the polymer to exhibit anisotropic properties in the melt phase. Such moieties may be formed from aromatic diols, aromatic amines, aromatic diacids; and aromatic hydroxy acids.
It further has been recognized that shaped articles formed from such liquid crystalline polymers may have their strengths enhanced by heating in a gaseous atmosphere below the melting temperature. See, for instance, in this regard U.S. Pat. Nos. 3,975,487; 4,161,470; 4,183,895; 4,247,514; and 4,424,184; and PCT International Publication No. WO 83/02424.
Poly(6-oxy-2-naphthoate-4-oxybenzoate) is recognized to be capable of forming an anisotropic melt phase which can be melt, extruded to form shaped articles such as fibers, films and three-dimensional molded articles. This wholly aromatic polyester and routes for its formation are disclosed in commonly assigned U.S. Pat. Nos. 4,161,470 and 4,393,191, and in commonly assigned U.S. Ser. No. 468,240,now U.S. Pat. No. 4,429,105 filed Feb. 22, 1983. It further has heretofore been recognized that shaped articles (e.g., fibers) formed from poly(6-oxy-2-naphthoate-4-oxybenzoate) commonly require many hours of heat treatment in a gaseous atmosphere in order to substantially increase the tenacity thereof. Such relatively long heat treatment time requirements have heretofore substantially extended the required processing times and have adversely impacted upon production economics.
It is an object of the present invention to provide an improved process for the formation of a high strength high modulus shaped article (e.g., fiber, film or three-dimensional molded article) of poly(6-oxy-2-naphthoate-4-oxybenzoate).
It is an object of the present invention to provide an improved process for the formation of poly(6-oxy-2-naphthoate-4-oxybenzoate) which has surprisingly been found to possess qualities which enable it to be more rapidly heat strengthened
It is an object of the present invention to provide an improved overall process for the formation of a high strength shaped article of poly(6-oxy-2-naphthoate-4-oxybenzoate) wherein the final heat strengthening step is substantially expedited.
It is an object of the present invention to provide an improved process for the formation of a multifilamentary fibrous material of poly(6-oxy-2-naphthoate-4-oxybenzoate) of high strength and modulus wherein undesirable coalescence or sticking between adjoining filaments is substantially avoided during the expedited heat strengthening step.
It is an object of the present invention to provide an improved process for the formation of a high strength shaped article of poly(6-oxy-2-naphthoate-4-oxybenzoate) wherein the polymer exhibits highly satisfactory heat stability characteristics during subsequent melt processing.
It is another object of the present invention to provide an improved process for the formation of a high strength shaped article of poly(6-oxy-2-naphthoate-4-oxybenzoate) wherein the productivity of a given production facility can be substantially increased because of the reduced time required to accomplish the heat strengthening step.
It is a further object of the present invention to provide an improved process for the formation of a high strength shaped article of poly(6-oxy-2-naphthoate-4-oxybenzoate) which requires a reduced capital investment to accomplish the formation of a specified quantity of product.
It is yet another object of the present invention to provide an improved process for the formation of a high strength shaped article of poly(6-oxy-2-naphthoate-4-oxybenzoate) which exhibits good conversion efficiency from the required raw materials to marketable product.
These and other objects, as well as the scope, nature and utilization of the claimed invention wlll be apparent to those skilled in the art from the following detailed description and appended claims.
It has been found that an improved process for the expeditious formation of a high strength shaped article of poly(6-oxy-2-naphthoate-4-oxybenzoate) which is capable of forming an anisotropic melt phase comprises:
(a) polymerizing under melt polymerization conditions in a polymerization zone at a temperature above 220.degree. C. approximately 10 to 90 mole percent of monomer I and approximately 10 to 90 mole percent of monomer II with the evolution of acetic acid by-product, wherein monomer I is: ##STR1## with the polymerization being conducted under a non-oxidizing gaseous atmosphere in the presence of a catalytic quantity of a potassium salt which is capable of catalyzing the polymerization, with the polymerization conditions including a polymerization temperature of at least 320.degree. C. and which exceeds the melting temperature of the polymer product by at least 20.degree. C. while operating at substantially atmospheric pressure until at least 95 percent of the stoichiometric quantity of acetic acid by-product is evolved and removed from the polymerization zone, and a subsequent polymerization temperature of at least 320.degree. C. and which exceeds the melting temperature of the polymer by at least 20.degree. C. while operating at a substantially reduced pressure of approximately 5 to 200 mm. of mercury to further increase the molecular weight of the polymer, PA1 (b) melt extruding the resulting polymer product to form a shaped article, and PA1 (c) substantially increasing the strength of the shaped article by heating in a non-deleterious gaseous atmosphere at a temperature below the melting temperature of the shaped article for two hours or less. PA1 (a) 31.1 grams of 6-acetoxy-2-naphthoic acid (0.135 mole), and PA1 (b) 65.8 grams of 4-acetoxybenzoic acid (0.365 mole).